This would store a function into the Y1 variable, which can act like a function. To call it, just do Y1(number), and that number will be stored into X and the function will be evaluated. Keep in mind that this method only works for functions that only take a single argument though. If you want a function that uses several arguments you will have to be more creative

To follow up on builderboy's post, if you have more than one variable that needs to be evaluated you can use 2nd+{7,8,9} to access u,v, and w. Not really sure what they are called but you can store equations to them and evaluate them like a function.

Yeah those characters work the same way as the function variables (in fact I think they are the.. differential equation variables?) but yeah you can do the same thing with Y1 and the rest :) Unfortunately this results in this being less in-line, but there is little way about it